Certain patients require the administration of two (and sometimes more) medications in liquid form. Diabetic patients who require two types of insulin, one short acting and the other long acting, are an example. Insulin injection is complicated by the fact that dosage requirements vary from patient to patient and even for the same patient. It is common practice to fill a single syringe with the required amounts of each type of insulin. One procedure presently used for doing this is as follows: the person administering the insulin injects into the first multiple dose vial (Vial #1), which contains one kind of insulin, a volume of air equal to the amount of insulin to be withdrawn from that vial. Then he/she withdraws the required amount of insulin from Vial #1 into the syringe. Then, after removing the needle from Vial #1, the person further withdraws the plunger to fill the syringe with the proper amount of air for injection into Vial #2, which contains a second type of insulin. The person inserts the syringe into Vial #2 and carefully expels the air in the syringe into Vial #2, taking care not to expel the solution from the syringe into Vial #2. Then the person withdraws the plunger, which causes the solution from Vial #2 to be drawn into the syringe. The syringe is then ready for injection. As one can appreciate, it is very difficult to expel the required amount of air in the Vial #2 without also expelling a small amount of insulin from Vial #1. If some insulin from Vial #1 is expelled, of course some insulin from Vial #1 will be introduced into the multiple dose Vial #2 so that Vial #2 no longer contains a pure solution of insulin of the second type. In other words, multiple dose Vial #2 becomes contaminated with insulin of the first type (i.e. from Vial #1) and is no longer pure.
An alternative procedure, seldom used, is simply to inject a solution of insulin of the first type (from Vial #1) from one syringe, then either inject the second type of insulin (from Vial #2) into the patient either using a second syringe or cleaning the first syringe after injection of the first type of insulin but before injection of the second. The alternative procedure has the advantage that neither of the multiple dose vials of insulin will become contaminated with the other type of insulin. It has the obvious and very serious disadvantage of requiring twice as many injections into the patient. Since the frequency of needle injections for diabetic patients is a problem at best, this mode of administration is unacceptable and is therefore seldom used.
The first generally used mode of administration will result in some contamination to at least one of the multiple dose vials of insulin with the other type of insulin unless the person doing the injections is very careful in withdrawing solutions from the respective multiple dose vials. The second mode described above is not widely used because it requires twice as many injections as the first.